The present invention relates to magnetic resonance imaging (MRI) and in particular to an interface for connecting local coils used in MRI imaging to an MRI machine.
Magnetic resonance imaging can provide sophisticated images of the human body by detecting faint nuclear magnetic resonance (“NMR”) signals, primarily from concentrations of hydrogen protons in the tissues of the body. In MRI, a patient is located in a strong, polarizing, magnetic field and hydrogen protons of the patient's tissues are excited into precession with a radio frequency (“RF”) pulse. A series of applied gradient magnetic fields are switched on and off to spatially encode the precessing protons by phase and frequency. A sensitive antenna is then used to detect the NMR signals which are reconstructed into images.
MRI machines normally provide an integral antenna as part of the magnet assembly that may be used both for the RF excitation pulse and for detecting the NMR signal. Preferably, however, the NMR signals will be detected using one or more “local coils” being one or more small antennas that may be positioned near the patient to provide for improved signal-to-noise ratio in the detection of the NMR signals.
Typically, a shielded cable is attached to the local coil to receive a signal from preamplifiers built into the local coil that amplify the signal before transmitting it to the MRI machine. The shielded cable may connect to a termination box on the MRI machine (a “dog house”) often at the end of the patient table, where signals from the shielded conductor are routed to the MRI processing electronics. The termination box may also provide a source of electrical power, transmitted through the shielded cable to the local coil, to power the preamplifiers. In addition, the shielded cable may conduct other electrical signals to the local coil including active decoupling signals communicating with decoupling circuits in the local coil to detune the local coil during the RF excitation pulse to prevent excessive current conduction in the local coil during that time period. The termination box may also provide a separate electrical connector for a second shielded cable passing to the local coil and conducting an RF excitation pulse to the local coil when the local coil operates both in a receive and transmit mode.
The area around the operating MRI machine represents a difficult electrical environment for connecting a local coil to the MRI acquisition circuitry, principally with respect to establishing a good radio frequency ground. The switched fields used during the imaging process can promote high shield currents on the shield that may cause heating and possible risk to the patient. Baluns, such as those described in U.S. Pat. No. 6,605,775 entitled: “Floating Radio Frequency Trap For Shield Currents” and hereby incorporated by reference and assigned to the assignee of the present invention, provide one method of reducing these shield currents.
The shielded cables passing from the local coils to the termination box are relatively bulky and inflexible, in part, as a result of the necessary physical separation required between the patient and currents in the shield (normally enforced by a thick insulator), and the inherent stiffness of the cable conductors. This later problem is exacerbated for multi-channel coils which employ separate conductors for each channel. The inflexibility and bulk of these shielded cables can cause storage problems when multiple coils must be stored on-site, for example, in the limited space of the MRI room.
One promising solution to the problems of shield currents and electrical interference is that of transmitting the NMR signals optically, for example, over optical fibers. However, this approach faces a number of practical problems. The first is the high cost of optical modulation circuitry suitable to provide high signal-to-noise transmission of the NMR signal, a cost that is multiplied by the number of channels of the local coil.
Optical connectors allowing connecting and disconnecting of the optical fiber system from the MRI machine are currently inadequate for use in the MRI environment and introduce unacceptable signal noise resulting from the extreme sensitivity of fiber connections to vibration induced changes in alignment.
Electrical power is still required by the optical modulator and/or preamplifier in the local coil, and cabling for this purpose offsets some of the benefit of increased flexibility of the fiber, as well as making any connector more complex, now having to handle optical and electrical signals.
One final problem with optical transmission of NMR signals from local coils is the large installed base of conventional local coils and MRI machines that are not “optically enabled”, accepting only electrical rather than optical signals. Such systems present an obstacle to large-scale adoption of an optical transmission system which initially would be suitable for only a small market of machines.